Road Log

Figure 1. Map of field trip localities in the area NW of Leavenworth, WA.

We plan to drive west on U.S. 2 from Wenatchee to Leavenworth. In Leavenworth, we will drive southwest along the Icicle Road [U.S.F.S. 76] to examine Chiwaukum Schist and the Mount Stuart batholith. We will return to Leavenworth then drive west on U.S. 2 and then north on WA 207 to Wenatchee Lake where we will see exposures of the Wenatchee Ridge Gneiss and Nason Ridge Migmatitic Gneiss. We will return to U.S. 2 and continue west to examine the Nason Ridge Migmatitic Gneiss and Mount Stuart batholith east of the Pacific Crest, then continue west across Stevens Pass to examine the Chiwaukum Schist in the contact aureole of the Mount Stuart batholith at Tunnel Creek.

Regional Geology of the Cascades Core

Figure 2. Geologic map of the Cascades Crystalline Core, WA. This compilation is based on 1:100,000 scale geologic mapping by the U.S. Geological Survey (e.g., Tabor et al., 1993).

The Cascades Crystalline Core [Cascades Core] is segmented by strike slip and normal faults into the Wenatchee [southwest] and Chelan blocks [northeast]. The western flank of the Cascades Core is truncated by the Straight Creek and Evergreen faults that separate low temperature and locally high pressure rocks of the Northwest Cascades thrust system from the Wenatchee Block. The eastern flank of the Cascades Core is covered by the Columbia River Basalts. In the northern Cascades Core, the high-angle Entiat fault divides the Wenatchee and Chelan blocks. In the south, these blocks are separated by the Tertiary Chiwaukum basin which formed southwest of the Entiat fault and is dominated by sedimentary rocks of the Chumstick Formation.

The Late Jurassic Ingalls Ophiolite crops out in the southernmost Wenatchee block. This low grade metamorphosed oceanic sequence is thrust over pelitic rocks of the medium grade Chiwaukum Schist along the Windy Pass thrust (Miller, 1985). This thrust fault is cut by ca. 95 Ma tonalite of the Mount Stuart batholith, constraining fault displacement to pre 95 Ma. North of the Mount Stuart batholith and beneath the Windy Pass Thrust, the dominant rock type is the Chiwaukum Schist. In the central Wenatchee block, this unit is intruded by a NW-SE trending belt of sills and dikes – these rocks are known as the Nason Ridge Migmatitic Gneiss. In the northern Wenatchee block, the medium to high grade quartzofeldspathic gneiss and amphibolite of the Swakane Gneiss and Napeequa unit are thrust over the Chiwaukum Schist along the White River shear zone.

Pseudomorphs of kyanite, garnet, staurolite and biotite after andalusite, garnet compositional zoning, and thermobarometric results have led numerous workers to conclude that pressure increased during metamorphism of the Chiwaukum Schist (e.g., Brown and Walker, 1993; Whitney et al., 1999). However, isochemical phase diagram sections and detailed garnet compositional zoning maps seem incompatible with the proposed pressure increase during garnet growth (Stowell et al., 2007). Several field localities and corresponding laboratory data will be examined in order to evaluate this issue.

Figure 3. Metamorphic mineral assemblages, mineral textures, and pressure-temperature-time [P-T-t] paths indicate that significant pressure increases [loading] occurred during metamorphism. This is evident in this P-T-t path for the aureole of the Mount Stuart batholith. A. (top) Kyanite, sillimanite, biotite and staurolite replacing andalusite. B. (bottom) P-T-t path for rocks along the northern margin of the batholith. Modified from Stowell and others (2007)

The Mount Stuart batholith has been important for understanding terrane amalgamation because it “stitches” the Ingalls Ophiolite (Miller, 1985) to the Chiwaukum Schist and has been an important and controversial source of paleomagnetic data for understanding the possible northward transport of terranes along the NW coast (e.g., Cowan et al., 1997). Recent work demonstrates that the batholith is composite in nature (Matzel et al., 2006) and we will discus the data and implications for this conclusion.

Field Trip Stop Descriptions

Stop 8-1: Chiwaukum Schist at the Mount Stuart Trailhead (UTM 0664136E; 5265883N)Windy Pass Thrust-Related Structures and Migmatitic Chiwaukum Schist
This field trip stop is in the southwest part of a large inclusion of Chiwaukum Schist within the Mount Stuart batholith. Outcrops along the fire road provide excellent examples of migmatitic schist and subhorizontal structures attributed to deformation on the Windy Pass thrust. Foliation is parallel to boudinaged quartz-rich layers and leucosomes, and is axial-planar to rootless fold hinges. Tourmaline-bearing, leucocratic dikes cut the foliation at high angles. The dominant metamorphic assemblage in the schist is garnet-biotite-plagioclase-quartz-cordierite-opaques. Whitney et al. (1999) reported temperatures of ~700 °C from these rocks, which are higher than those recorded in most of the thermal aureole of the Mount Stuart batholith. This is compatible with the development of the stromatic migmatites and may have led to thermal weakening of the schist during Windy Pass thrusting. Leucosomes have thin selvages of cordierite and biotite; the cordierite replaces biotite, may be peritectic, and locally contains inclusions of sillimanite and hercynite. In the mesosome, garnet inclusion trails are crenulated, with axial planes parallel to matrix foliation. This foliation wraps the garnet porphyroblasts, and strain shadows are well developed. Garnet is inferred to have grown before or early during formation of thrust-related foliation.

Optional Stop (UTM 0665016E; 5267279N)
Park on left across from brown U.S. Forest Service post labeled 7601. Walk up the closed logging road ~50 m.Mount Stuart Fabrics, Geochronology, and the Windy Pass Thrust

This outcrop illustrates the strong, gently dipping magmatic foliation with weak subsolidus overprint that characterizes the Mount Stuart batholith near the Windy Pass thrust and a large (2.2 km2), strongly deformed inclusion of Chiwaukum Schist. Movement on the thrust likely formed the intense foliation while melt was present, and solidification of the batholith may have led to cessation of thrust movement and partitioned shortening elsewhere into the weaker schist (Miller and Paterson, 1994). This stop lies at the northern contact of the schist inclusion with the structurally overlying batholith. In the inclusion, the schist is commonly migmatitic. Foliation in the schist is axial-planar to folds of compositional layers, and weak lineation trends approximately E-W. Lineation is also weak in the tonalite, compatible with the flattening fabric in this domain. Foliation in the tonalite is discordant to the contact but subparallel to foliation in the schist, supporting evidence throughout the batholith that magmatic foliation formed late during crystallization and records strain, not flow parallel to the margin of the body. A tonalite similar to that at this outcrop collected 500 m to the west yielded a U-Pb zircon age of ca. 94.4 Ma (Matzel et al., 2006).

Stop 8-2: Chiwaukum Schist at Chatter Creek, Icicle Canyon (UTM 0658946E; 5274579N)Structures, Metamorphism, and Ultramafic Blocks in Chiwaukum Schist
This section of the Chiwaukum Schist is a short distance southwest of the central sill-like region of the Mount Stuart batholith and well below the Windy Pass thrust. Stream-polished surfaces (be careful!) directly west of the footbridge on the north bank of Icicle Creek illustrate styles of folding in multiply deformed quartz-mica-garnet-fibrolite schist. Overprinting relations (e.g., crosscutting cleavages and refolded folds or cleavages) preserve evidence for at least two transposition cycles, which Miller and Paterson (1994) relate to pre- to syn-emplacement regional NE-SW shortening. Note the subhorizontal, NW-SE–trending mineral lineation and fold axes, asymmetry of folds, and north-dipping axial planes that shallow with decreasing inter-limb angle. Most of the youngest folds and some of the older folds have asymmetries indicating top-to-SW motion. Older quartz-rich layers are folded, whereas quartz-feldspar-rich “leucosomes” and garnet-bearing dikes cut most of the older structures, but are locally deformed. Paterson et al. (1994) reported a temperature of 685°C from biotite-garnet schist at this locality, and a sample near this stop yielded a K-Ar biotite age of 85 Ma (Engels and Crowder, 1971). Return to the main road and walk up this road (northwest) 25 m to the first roadcut where garnet-bearing dikes cut the multiply deformed Chiwaukum Schist. Similar tonalite dikes found throughout the southern Nason terrane have plagioclase coronas around single or multiple garnet grains (Stowell and Stein, 2005). Garnet zoning, and plagioclase zoning in the coronas and matrix, are compatible with garnet growth, consumption of early high-An plagioclase, and growth of lower-An plagioclase during prograde metamorphism. This is in marked contrast to earlier interpretations of garnet breakdown to plagioclase due to high-T decompression. Foliation is steep within a few hundred meters of the Mount Stuart contact, dips moderately northeast in the central Icicle Canyon area (e.g., this locality), and gradually flattens to near horizontal dips near the Windy Pass thrust. Also note that from the road we can see orange weathering metaperidotite in the hanging wall of the Windy Pass thrust on the distant, high ridge to the southwest. Walk back down (southeast) the road and climb east up the embankment of the turnoff to Chatter Creek Campground. On the knoll a few tens of meters north of the road (UTM 0659014; 5274605) are blocks of metaperidotite, encased within schist, which are part of a ~300-m-wide, E-W–trending belt that passes through the campground and has been traced for ~1.1 km to the east. Typical mineral assemblages include forsterite-tremolitetalc (amphibolite facies). Amphibolites are also common in and near this belt and may be related to the ultramafites. The relationship of the ultramafi tes in the southern part of the Chiwaukum Schist to the pelitic schists can be explained by at least three hypotheses, which are discussed in detail in Paterson et al. (1994): (1) The ultramafites are imbricate slices and/or infolded klippen of the Ingalls Complex, presumably associated with the Windy Pass thrust; (2) The ultramafites represent serpentinite blocks that slid from an uplifted mass of the Ingalls Complex into the sedimentary protoliths of the schist (Tabor et al., 1987), implying that the protoliths are Late Jurassic or Early Cretaceous; (3) The ultramafites are not part of the Ingalls Complex, but are slices of oceanic mantle imbricated with the Chiwaukum clastic protolith, perhaps in an accretionary wedge before or during metamorphism.

Figure 4. Chiwaukum Schist at Chatter Creek in Icicle Canyon.

Stop 8-3: Mount Stuart batholith on the Wenatchee River, west of Leavenworth (UTM 0671883E; 5273921N)Igneous Textures in Mount Stuart Batholith
At this locality we will examine homogenous tonalite of in the easternmost ‘keel’ of the Mount Stuart batholith.

Stop 8-4: Wenatchee Ridge Gneiss at the Overlook (UTM 0659240E; 5302301N)Wenatchee Ridge Gneiss and Overview of the Chiwaukum Graben
East of Lake Wenatchee, Entiat Ridge with the Eocene Leavenworth fault at its base and Dirtyface Mountain block the horizon. Looking southwest, the Little Wenatchee River separates Nason Ridge to the southwest of Lake Wenatchee from Wenatchee Ridge, immediately to the north. Regional upright, gently plunging folds occur throughout this region, with the hinge line of a major antiform passing through the lake and along Wenatchee Ridge. The axis of a smaller synform occurs along the Little Wenatchee River and another antiform axis lies along Nason Ridge. The Dirtyface pluton is exposed in the upper part of Dirtyface Mountain in the northeast limb of the Wenatchee Ridge antiform. Miller and others (2003) qualitatively unfolded these regional folds to construct a crustal section.
Wenatchee Ridge Gneiss – Rob Holler. The Wenatchee Ridge Gneiss is lowest structural unit in the Wenatchee block and may represent the base of the crust currently exposed in the Wenatchee block. A U-Pb zircon age determined by multi collector – laser ablation ICPMS from this locality yield an age of 88.9±2.5 Ma and younger zircon rims of ca. 85 Ma (Holler & Stowell, unpublished) suggesting emplacement simultaneous with metamorphism of the adjacent NRMG. Garnet amphibolite and pelite in the NRMG yield metamorphic pressures of 8-10 kbar. The small outcrop at the overlook is representative of the heterogeneous Wenatchee Ridge Gneiss. Within walking distance of the parking area are a variety of lithologies and deformational fabrics. Outcrops to the north and southeast expose tonalitic gneiss first described by van Diver (1967). The dominant mineral assemblage is biotite-muscovite-plagioclase-quartz±opaques. To the east of the overlook parking area two trails lead to exposures of a heavily altered pod of soapstone and an outcrop of strongly deformed gneiss. This intensely foliated gneiss contains garnet-amphibole-potassium feldspar-opaques-biotite-plagioclase-quartz. The outcrop offers excellent three dimensional exposures of small-scale folding and lineations. A nearby blast pit exposes asymmetric, overturned, plunging folds with axes plunging 53º -145º. In addition, there is a small outcrop of amphibolite a short distance along the trail.

Figure 5. Lake Wenatchee as seen looking SE from the overlook at the SE end of Wenatchee Ridge. Dirtyface Mountain frames the left side of the image, Entiat Ridge can be seen behind the lake, and Nason Ridge frames the image on the right or west.

Optional/Alternate –Lake Wenatchee
Looking west across the Eocene Leavenworth fault we see an overview of the regional structure and rock types in the eastern Wenatchee block. Across the lake, the Little Wenatchee River separates Nason Ridge to the southwest of Lake Wenatchee from Wenatchee Ridge, immediately to the west. The White River drainage and Dirtyface Mountain lie to the northwest. Regional upright, gently plunging folds occur throughout this region, with the hinge line of a major antiform passing through the lake and along Wenatchee Ridge. The axis of a smaller synform occurs along the Little Wenatchee River and another antiform axis lies along Nason Ridge. The Dirtyface pluton is exposed in the upper part of Dirtyface Mountain in the northeast limb of the Wenatchee Ridge antiform. Much of the eastern end of Nason and Wenatchee Ridges consists of the Wenatchee Ridge Gneiss and overlying gneisses. Structurally higher areas (e.g., Dirtyface Mountain and northwest) are dominated by Chiwaukum Schist intruded by tonalite orthogneiss sheets. The Tenpeak pluton is located in a separate structural domain north and east of Dirtyface Mountain.Optional –Ten Peak Pluton and Dikes along the White River

Stop 8-5: Nason Ridge Migmatitic Gneiss on U.S. 2 (UTM 0657900E; 5294372N)Nason Ridge Migmatitic Gneiss
Biotite schist and amphibolite of the Chiwaukum Schist are intruded by numerous dikes and sills in this outcrop, which is part of a regional injection zone of tonalite and trondhjemite into the Chiwaukum Schist. This zone is called the Nason Ridge Migmatitic Gneiss (Tabor et al., 2002). The outcrop lies ~3 km northeast of the Mount Stuart batholith. It is dominated by dikes and sheets; amphibolite and biotite schist are most abundant at the west end of the outcrop. The dominant assemblage in the pelitic schist is garnet-kyanite-biotite-quartz-plagioclase-muscovite, and staurolite is found locally (Magloughlin, 1994). Magloughlin (1994) calculated 6.5 kbar and 572 °C from this outcrop. The Chiwaukum Schist has been multiply folded; a prominent hinge of folded foliation in amphibolite is particularly well displayed on the south side of the highway. The intrusive sheets are texturally variable, range from diorite to granite, and are dominantly tonalitic. Rb-Sr and Sm-Nd analyses suggest that some of the compositional variability results from contamination by the biotite schist (Magloughlin, 1994). The sheets also display variable orientations and crosscutting relations. Most lack subsolidus foliation, but some are folded or boudinaged and have strong, gently plunging magmatic and locally subsolidus mineral lineation. Numerous small faults and a steep, low-temperature shear zone are also present. Overall, the sheets record less subsolidus deformation and recrystallization than to the northeast, in the Nason terrane. Paterson et al. (1994) inferred that the sheets were emplaced during the latest stages of NE-SW shortening and NW-SE extension in the southern part of the Wenatchee block. U-Pb zircon ages of 84–92 Ma (Stowell unpublished; Walker and Brown, 1991) from northwest of this outcrop in the Nason Ridge Gneiss suggest that at least some of the tonalite sheets are younger than the Mount Stuart batholith. Age constraints for this outcrop are a biotite Ar/Ar date of 83 ± 1 Ma (S.A. Bowring, in Paterson et al. (1994)) from a tonalite sheet, Rb-Sr whole-rock muscovite dates of 73 ± 1 Ma and 83 ± 1 Ma from two pegmatites (Magloughlin, 1994).The significance of the sheets remains enigmatic. The multiple magmatic pulses may mark unfocused magma pathways or represent the plumbing system for a larger pluton, but this is unlikely to be the Mount Stuart batholith, given the apparently different ages. Brown and Walker (1993) postulated that the sheets were intruded at the margin of a now eroded batholith, which was emplaced above, and caused the loading recorded by the metamorphism of the Chiwaukum Schist.

Stop 8-6: Mount Stuart batholith on U.S. 2, east of Stevens Pass (UTM 0646792E; 5294096N)Mount Stuart Batholith
These roadcuts are typical of the hook-shaped region of the northeast body of the batholith. Hornblende-biotite tonalite (this outcrop) contains microgranitoid enclaves and has moderately strong magmatic foliation and weak lineation defined best by hornblende and plagioclase. In this area, the foliation is folded by decameter- to kilometer-scale magmatic folds (Paterson and Miller, 1998). The outer 1–2 km of the batholith to the north of this stop commonly contains peraluminous garnet- and muscovite-bearing biotite tonalite and granodiorite, unlike the hornblende-bearing tonalite in most of the batholith. Whole rock samples from near the margin exhibit elemental variations off the main batholith trends and have elevated delta 18O, indicating variable but significant wall rock interaction (Paterson et al., 1994). Several zircon grains from this outcrop, analyzed by TIMS U-Pb geochronology, contained slightly older inherited cores, but the tip of one grain yielded a concordant analysis with a 206Pb/238U date of 96.2 Ma (Matzel et al., 2006). Zircon grains from nearby outcrops of granodiorite also contain inherited cores, but concordant analyses yield an age that overlaps within error of the age obtained from this outcrop (Matzel et al., 2002a). New LA-MC-ICPMS 206Pb/238U ages from Union Gap (NW) and Rapid River (W) indicate that this entire section of the batholith is 96-97 Ma and that zircon from some older ca. 100-102 Ma magma was incorporated (Stowell & Gatewood unpublished). Engels and Crowder (1971) reported K-Ar ages of 95 Ma from hornblende and 82 Ma from biotite in the batholith near this locality.

Stop 8-7: Andalusite bearing Chiwaukum Schist U.S. 2, Tunnel Creek (UTM 0641689E; 5286346N)Chiwaukum Schist in the Aureole of the Mount Stuart Batholith
This stop is in a narrow septum of Chiwaukum Schist between the southwest and northeast bodies of the Mount Stuart batholith. We will examine andalusite-bearing mineral assemblages and syn-emplacement structures in the contact metamorphic and structural aureole of the batholith. Outcrops contain folded andalusite + garnet + biotite schist typical of the Chiwaukum Schist exposed adjacent to the northern contact of the batholith. Most rocks display steep foliation. Fold axes and mineral lineations are variably oriented in the septum and regionally have gentle northwest or southeast plunges, but a steep andalusite lineation prevails here. Felsic dikes that are thought to be related to the batholith show significant deformation and recrystallization. Based on oxygen isotopes, plutonic rocks on the northeast side of the screen exhibit variable degrees of host rock interaction. Engels and Crowder (1971) reported K-Ar ages of 85 Ma from both hornblende and biotite from nearby, suggesting rapid cooling. This screen of Chiwaukum Schist is less than 1 km wide and lies entirely within overlapping contact aureoles of the southwest and northeast bodies of the Mount Stuart batholith. Spectacular andalusite porphyroblasts, up to several centimeters in length display chiastolite crosses, are found with smaller porphyroblasts of staurolite and garnet. The andalusite is near pristine (unlike much andalusite in the Wenatchee block), but some crystals are replaced by sillimanite or staurolite. Fibrolite also replaced biotite as aligned crystals in the necks of boudinaged andalusite crystals and in bundles overprinting the foliation (Evans and Berti, 1986). Euhedral staurolite crystals locally replaced andalusite or grew across the matrix foliation. Much of the andalusite in this part of the Wenatchee block has been replaced by sillimanite, kyanite, and/or staurolite that grew during regional metamorphism that postdates the Mount Stuart batholith. Stowell and others (2007) report a quantitative P-T-t path and a Sm-Nd garnet age of 86.1±1 Ma for metamorphism of similar rocks adjacent to and north of the Mount Stuart batholith. Rocks in this area also experienced andalusite zone metamorphism synchronous with batholith emplacement. Garnet growth initiated during subsequent, regional metamorphism at ~4.8 kbar and 595°C and culminated at ~6.0 kbar and 670°C.